1. Field of the Invention
This invention relates generally to aircraft automatic braking systems, and particularly relates to a hydraulic brake valve control system for use in an aircraft wheel braking system, utilizing a primary brake valve control subsystem and a backup brake valve control subsystem.
2. Description of Related Art
Conventional aircraft braking systems typically have a normal operating mode, either under pilot or computer control, and a backup or override system. Such backup systems can consist of a full control system, similar to the normal system, or a simplified system supplied by a hydraulic accumulator or air bottle. The degree of complexity and extent of duplication of the normal operating control system that is practical depends largely upon the weight and cost of the aircraft.
Large commercial passenger transport, for example, normally have a complete dual system. Such primary and backup systems are commonly isolated by a "relay valve" which permits only one system to be in command at any given time. If, for some reason, there is a deterioration in the primary system, the pilot or automatic control can switch to the backup system, and the relay then switches over, to permit braking to be controlled through the backup system. As an alternative to hydraulic switching large electrical shut off valves can be used to isolate one or more brake lines. These shut off valves are typically incorporated in brake-by-wire systems, where computer control is the primary source of braking commands. The electrical shut off valves are large in order to carry the full flow of anti-skid driven brake lines. The design of such valves is often further complicated when they must handle two to four brake lines. Computerized and microprocessor based brake control systems are known from U.S. Pat. Nos. 4,402,047 and 4,749,238, for example. Automatic braking systems for aircraft utilizing a logic control circuit and analog control systems, and a metering valve with a metering spool sliding between on and off positions are generally known from U.S. Pat. No. 4,120,540. U.S. Pat. No. 4,591,213 discloses a braking system which switches between two different primary and secondary subsystem, based upon temperature and friction sensed at the brakes, rather than based upon failure of one of the subsystems.
It would be desirable to provide a brake valve control system for aircraft that eliminates the need for a relay valve or large electrical shut off valves, and also reduces the weight and cost of providing a backup braking system. The present invention addresses these needs.